The in situ Fourier transform infrared spectroscopic study of isotactic polypropylene (iPP) shows that a structure change occurs at about 135 degreesC. Some experimental results suggest that this structure change may result from the disordering of helical sequences in the noncrystalline region, which can be further confirmed by the in situ infrared observation of chloride polypropylene (Cl-PP). If the iPP sample is drawn, the transition temperature is elevated. According to the change of infrared absorbance with temperature between 135 and 155 degreesC, the various regularity bands of iPP can be rearranged in terms of the order degree from high to low: 940, 1220, 1167, 1303, 1330, 841, 998, 900, 808, 1100 and 973 cm(-1). This sequence is identical with our recent observation based on the melting of iPP crystals.